reserve w, w1, w2 for Element of ExtREAL;
reserve c, c1, c2 for Complex;
reserve A, B, C, D for complex-membered set;
reserve F, G, H, I for ext-real-membered set;
reserve a, b, s, t, z for Complex;
reserve f, g, h, i, j for ExtReal;
reserve r for Real;
reserve e for set;

theorem Th20:
  --{a,b} = {-a,-b}
proof
  thus --{a,b} = --({a}\/{b}) by ENUMSET1:1
    .= --{a} \/ --{b} by Th15
    .= {-a} \/ --{b} by Th19
    .= {-a} \/ {-b} by Th19
    .= {-a,-b} by ENUMSET1:1;
end;
